Squamosal bone

A schematic of asynapsidskull showing the location of major dermal bones, including the squamosal bone (Sq).

See also:Evolution of mammalian auditory ossicles

Thesquamosalis askullbone found in mostreptiles,amphibians,andbirds.In fishes, it is also called the pterotic bone.

In most tetrapods, the squamosal andquadratojugalbones form the cheek series of the skull.The bone forms an ancestral component of the dermal roof and is typically thin compared to other skull bones.

The squamosal bone liesventralto the temporal series and otic notch, and is bordered anteriorly by thepostorbital.Posteriorly, the squamosal articulates with thequadrateandpterygoidbones. The squamosal is bordered anteroventrally by thejugaland ventrally by thequadratojugal.

Function in reptiles

In reptiles, thequadrateandarticularbones of the skull articulate to form the jaw joint. The squamosal bone lies anterior to the quadrate bone.

Anatomy in synapsids

Non-mammaliansynapsids

In non-mammaliansynapsids,the jaw is composed of four bony elements and referred to as a quadro-articular jaw because the joint is between thearticularandquadrate bones.Intherapsids(advanced synapsids including mammal), the jaw is simplified into an articulation between thedentaryand thesquamouspart of thetemporal bone,and hence referred to as a dentary-squamosal jaw.

Mammals

In many mammals, including humans, the squamosal fuses with theperiotic boneand theauditory bullato form thetemporal bone,then referred to as thesquama temporalis.

In mammals, thequadratebone evolves to form theincus,one of theossicles of the mammalian ear.Similarly, thearticularbone evolves to form themalleus.The squamosal bone migrates and lengthens to become a new point of articulation with the lower jaw (at thedentarybone).